Method for measuring blood oxygen content under low perfusion

ABSTRACT

A method for measuring blood oxygen content under low perfusion, which is used in a device for measuring blood oxygen content, includes the steps of: initializing the device that is applied with power, collecting and processing data with a driving circuit of light emitting device, a bias circuit, a gain circuit and an A/D sampling circuit, which are controlled under a core control module; calculating blood oxygen saturation based on the collected data with a data processing module which integrates the collected data in a period of time with an area integration method; and outputting from a communication functional module results of the blood oxygen saturation or pulse rate calculated with the data processing module. The method further includes a decision step of deciding the two results acquired from the data processing module with the waveform method and the integration method respectively based on the intensity of the measured signal and generating the final measured result, performed by a decision unit included in the device. By adopting the above method, the disturbance to effective signal by noise can be eliminated. As a result, the measuring accuracy of blood oxygen content under low perfusion can be improved without increasing the production cost for the measuring device.

FIELD OF THE INVENTION

The present invention relates to medical instruments, more particularlyto a device for measuring blood oxygen saturation, and especially to amethod for measuring blood oxygen content under low perfusion.

BACKGROUNGD OF THE INVENTION

It is very necessary to monitor the state of blood oxygen for patientsin the process of operation and reablement, generally, by monitoring aparameter of blood oxygen saturation. Conventionally, the aboveparameter is measured with spectrophotometry which utilizes thedifference between light absorption coefficients of reduced hemoglobinand oxyhemoglobin based on the Lambert-Beer law and the theory of lightscattering. The spectrophotomety can be performed by transmitted lightor reflected light. The Lambert-Beer law is expressed as:I=I₀e^(−εcd),Where I is the intensity of transmitted light, I₀ is the intensity ofincident light, C is the concentration of the light-receiving matter insolution, d is the path length of light absorbed by solution, and ε isthe light absorption coefficient of the matter. From the above equation,the absorbance D is reached as follows:D=lnI ₀ /I=εcd.It indicates that the light absorption of the matter correlates with theconcentration thereof, which implies the possibility of calculatinginternal composition of tissues from the light absorption of them.

The researchers have further researched the reduced hemoglobin (Hb) andthe oxyhemoglobin (HbO₂) closely correlating with the blood oxygensaturation. It is found that the difference between the light absorptioncoefficients of HbO₂ and Hb is notable, as shown in FIG. 2, in which thesolid line represents the light absorption coefficient-wavelength curveof HbO₂, and the dotted line represents the light absorptioncoefficient-wavelength curve of Hb. It is shown in FIG. 2 that the lightabsorption coefficient of HbO₂ is only one tenth ( 1/10) of that of Hbfor the visible red light with wavelength of 660 nm, but the lightabsorption coefficient of HbO₂ is greater than that of Hb for theinfrared light with wavelength of 940 nm, and the light absorptioncoefficients of HbO₂ and Hb have one isoabsorption point for theinfrared light with wavelength of 805 nm.

The arterial blood oxygen saturation is defined as:SaO₂ =HbO₂/(Hb+HbO₂)=C ₁/(C ₁ +C ₂),  (1)where C₁ is the concentration of HbO₂, and C₂ is the concentration ofHb. SinceD(660)=lnI ₀(660)/I(660)=ln(I ₀(660)/I(660)e ^(−ε) ¹ ^(c) ¹ ^(d) e ^(−ε)² ^(c) ² ^(d))=ε₁ c ₁ d+ε ₂ c ₂ d,  (2)D(805)=lnI ₀(805)/I(805)=ln(I ₀(805)/I(805)e ^(−ε) ³ ^(c) ¹ ^(d) e ^(−ε)⁴ ^(c) ² ^(d))=ε₃ c ₁ d+ε ₄ c ₂ d,  (3)where ε₁ and ε₂ are the light absorption coefficients of HbO₂ and Hb forthe red light with wavelength of 660 nm respectively, ε₃ and ε₄ are thelight absorption coefficients of HbO₂ and Hb for the infrared light withwavelength of 805 nm respectively and both equal to ε (i.e. ε₃=ε₄=ε),and d is the thickness of the light-transmitting tissue, the followingequations can be reached:C ₁ +C ₂ =D(805)/εd,C ₁=(D(660)−ε₂ D(805)/ε)/(ε₁−ε₂)d.By substituting them into the equitation (1), the following equation isreachedSaO₂ =A×D(660)/D(805)+B,  (4)where A=ε/(ε₁−ε₂) and B=ε₂/(ε₁−ε₂).

However, D(660) and D(805) are not only relevant to Hb and HbO₂, asexpressed in the equations (2) and (3), but also relevant to theabsorption of muscles, bones, pigments, adiposes, venous blood and thelike in tissues. That is, each of D(660) and D(805) should furtherinclude a portion of background absorption as shown in FIG. 3, so theequations (2) and (3) becomeD(660)=lnI ₀(660)/I(660)=ln(I ₀(660)/I _(B) e ^(−ε) ¹ ^(c) ¹ ^(Δd) e^(−ε) ² ^(c) ² ^(Δd))  (5)D(805)=lnI ₀(805)/I(805)=ln(I ₀(805)/I _(B) e ^(−ε) ³ ^(c) ¹ ^(Δd) e^(−ε) ⁴ ^(c) ² ^(Δd))  (6)where I₀ is the intensity of incident light, I_(B) is the intensity oftransmitted light when only the background absorption of tissuespresents, Δd is the variation of the transmission distance as a resultof the change from blood-free to blood-perfused. The backgroundabsorbance is easily defined as:D _(B) =ln(I ₀ /I _(B)).Thereby, the following equations can be reached:D(660)−D _(B)(660)=ε₁ C ₁ Δd+ε ₂ C ₂ Δd,  (7)D(805)−D _(B)(805)=ε₃ C ₁ Δd+ε ₄ C ₂ Δd,  (8)where ε₃=ε₄=ε, so the equation (4) becomeSaO₂ =A×(D(660)−D _(B)(660))/(D(805)−D _(B)(805))+B.  (9)The equation (9) is the fundamental formula for detecting the bloodoxygen saturation.

Generally, the infrared light with one isoabsorption point forwavelength of 805 nm is not utilized to detect the blood oxygensaturation, because it is hard to acquire the precise value of suchwavelength and resultantly relatively large error occurs. The infraredlight with wavelength of about 940 nm is commonly utilized, for thereason that the variation of the light absorption coefficients of HbO₂and Hb for the wavelength around are more smooth and thus little errorusually occurs. When the infrared light with wavelength of 940 nm isutilized, since ε₃ is not equal to ε₄ (i.e. ε₃≠ε₄) in the equation (8),the equation (9) becomes the blood oxygen saturation Spo₂Spo ₂=(A×R+B)/(C×R+D),  (10)where A=ε₁, B=−ε₂, C=ε₄−ε₃, D=ε₁−ε₂, and

$\begin{matrix}{R = {\frac{{D(660)} - {D_{B}(660)}}{{D(940)} - {D_{B}(940)}}.}} & (11)\end{matrix}$It can be known from the above equations that “R” and blood oxygensaturation are one to one correspondence. Since D=LnI₀/I=εcd,

$\begin{matrix}{{R = {\frac{{\ln\mspace{11mu}{I_{R\; 0}/I_{RM}}} - {\ln\mspace{11mu}{I_{R\; 0}/I_{Rm}}}}{{\ln\mspace{11mu}{I_{I\; 0}/I_{IM}}} - {\ln\mspace{11mu}{I_{I\; 0}/I_{Im}}}} = \frac{\ln\mspace{11mu}{I_{Rm}/I_{RM}}}{\ln\mspace{11mu}{I_{Im}/I_{IM}}}}},} & (12)\end{matrix}$where I_(RM) is the maximum intensity of the transmitted light of redlight, I_(Rm) is the minimum intensity of the transmitted light of redlight, I_(R0) is the intensity of the incident light of red light,I_(IM) is the maximum intensity of the transmitted light of infraredlight, I_(Im) is the minimum intensity of the transmitted light ofinfrared light, and I_(I0) is the intensity of the incident light ofinfrared light. With regard to red light, the following equation can bereached:

$\begin{matrix}{{\ln\mspace{11mu}{I_{Rm}/I_{RM}}} = {{\ln\left( {1 - \frac{I_{RM} - I_{Rm}}{I_{RM}}} \right)}.}} & (13)\end{matrix}$When the ratio of pulsating component to direct current (DC) component,namely (I_(RM)−I_(Rm))/I_(RM) is small,

${\ln\left( {1 - \frac{I_{RM} - I_{Rm}}{I_{RM}}} \right)} \approx \frac{I_{RM} - I_{Rm}}{I_{RM}} \approx \text{pulsating~~component/DC~~component.}$Accordingly, R can be expressed as follows:

$\begin{matrix}{R = \frac{{Red}_{AC}/{Red}_{DC}}{{Ir}_{AC}/{Ir}_{DC}}} & (14)\end{matrix}$where Red_(AC) is the alternating current (AC) component of theintensity of transmitted red light (i.e. AC peak value of the intensityof red light), Red_(DC) is the DC component of the intensity oftransmitted red light, Ir_(AC) is the AC component of the intensity oftransmitted infrared light (i.e. AC peak value of the intensity of theinfrared light), and Ir_(DC) is the DC component of the intensity oftransmitted infrared light. From the above equations, it can be seenthat the main factor influencing the variable R is the AC components ofthe intensity of transmitted red light and infrared light, because theDC components of the intensity of the two transmitted lights arerelatively stable for a period of time after the operating state of thelight emitting diode is adjusted and fixed. Now the AC component iscalculated by finding out the maximum value and minimum value of theintensity of the two transmitted lights. Therefore, the value of “R” canbe calculated if the waveforms of the two transmitted lights in a fullpulse wave were known.

In a human body, the arterial blood pulsates in the end parts of tissuesas a result of the pulse wave, and the HbO₂ and Hb cause the end partsof tissues (such as fingers) to have different transmittivities for redlight and infrared light. Nowadays, according to the above principle,the domestic or foreign pulse oximeters operate by irradiating red lightand infrared light with a certain intensity to the fingers, detectingthe transmitted light intensities of the two lights, and thencalculating the blood oxygen saturation based on the ratio of thedensity variations of the red light and the infrared light after the twolights passing through the fingers and the corresponding equationsdescribed above.

According to the principle described above, a device for measuring bloodoxygen saturation basically includes a blood oxygen sensor and a signalprocessing unit The key element of the blood oxygen sensor is a sensorincluding a light-emitting diode (LED) and a photosensor. The LED canprovide the lights of two or more wavelengths. The photosensor canconvert the light signals passing through the fingers and containing theinformation of blood oxygen saturation into electrical signals which areprovided to a signal processing module to be digitalized for calculatingthe blood oxygen saturation.

More particularly, the measuring device can be functionally divided intothe following parts, i.e. a power supply circuit, a driving circuit, asignal amplifying and processing part, an A/D (analog/digital)converting circuit, a logical control part, a single-chip microcomputerdata processing part and the like. Specifically, as shown in FIG. 1, themeasuring device includes: a power supply circuit which outputs twogroups of power supply to the whole measuring device, wherein one is +5Vfor digital circuit and the other one is ±5V for analog circuit, whilean AC or DC power supply of ±12V is input; a driving circuit adjusted bythe logical control part to output currents with different amplitudesfor driving the LED, in order to ensure that a light-receiving device(for example photocell) can output signals with certain amplitudes; asensor part detecting the light signals passing through the fingers andthen converting the light signals into electrical signals which aretransmitted to a signal amplifying and processing part; the signalamplifying and processing part which applies differential amplifyingprocess, background photocurrent cancel process, gain adjusting and biascurrent cancel process to the electrical signals and transmits theelectrical signals so-processed to an A/D converting circuit; the A/Dconverting circuit converting the electrical signals to digital signalswhich is transmitted to a single-chip microcomputer to be processed; thesingle-chip microcomputer data processing part whose calculation modulesimulates and analyzes the waveform based on the sampled signals to findout the maximum value and minimum value of pulse waveform and thencalculates the peaks value of the pulse waveform and the blood oxygensaturation; a serial port circuit through which the parameters of pulsewave described above and the blood oxygen saturations isolated byoptical coupler are transmitted, wherein the logical control part isutilized to make various parts under the controls of the single-chipmicrocomputer, such as the control over light-emitting sequential of thesensor, the control over driving current the control over bias currentthe control over background light cancel, the control of signals A/Dconverting and the like.

However, there are the following disadvantages in the conventionalmethod described above. The level of perfusion is usually very low forpatients. Since it is necessary to measure the AC component of the pulsewaveform under this condition, that is, to find out the maximum valueand minimum value of the waveform, but the signal to be measured is verypoor under low perfusion and the signal-to-noise ratio (SNR) is very lowas well, it becomes difficult to find out the waveform. Therefore,errors may occur during the process of measuring the peak value of thepulse wave, and the ratio of AC to DC obtained thus may be wrong, whichcause the value of blood oxygen content measured finally to have verylow accuracy.

SUMMARY OF THE INVENTION

The present invention intends to provide a method for measuring bloodoxygen content, which can accurately measure or monitor the value ofblood oxygen content by analyzing and calculating the sampled data ofpulse wave, as concerning the case where the level of perfusion is lowand signals are poor.

To solve the problem described above, the general concept of the presentinvention is proposed as follows. Since it can be proved that theintegration result of the sampled data of pulse wave corresponds to ACcomponent of the pulse wave, the peak value of waveform, which isnecessary to be found in the conventional method, can be replaced by thearea integration of signal to calculate the blood oxygen saturation.Thus, only it is necessary to integrate the pulse waveform in a periodof time. Further, the disturbance to effective signal by noise can beeliminated with the integration of the noise in the period of timeapproximating to zero. Therefore, the measuring accuracy of blood oxygencontent under low perfusion can be improved.

As the technical solution for realizing the general concept of thepresent invention, there is provided a method for measuring blood oxygencontent under low perfusion, which is used in a device for measuringblood oxygen content, includes the steps of:

a. initializing the device that is applied with power,

b. collecting and processing data with a driving circuit of a lightemitting device, a bias circuit, a gain circuit and an A/D samplingcircuit, which are controlled under a core control module;

c. calculating blood oxygen saturation based on the collected data witha data processing module which integrates the collected data in a periodof time with an area integration method; and

d. outputting from a communication functional module results of theblood oxygen saturation or pulse rate calculated with the dataprocessing module. fits envelope waveform of a pulse wave

In the step of c, the data processing module further fits envelopewaveform of the pulse wave and finds out the maximum value and theminimum value of the pulse wave to calculate the blood oxygen saturationwith a waveform method based on the collected data. The method, betweenthe steps of c and d, further includes a decision step of deciding thetwo results acquired from the data processing module with the waveformmethod and the integration method respectively based on the intensity ofthe measured signal and generating the final measured result, performedby a decision unit included in the device. The device performs thesampling and measuring by making at least two lights pass through endparts of tissues, wherein one is red light, and the other is infraredlight; and in the step of c, the data of the two lights is integratedrespectively to calculate the ratio of integration result of the redlight to that of the infrared light for replacing the ratio of AC peakvalue of the intensity of the red light Red_(AC) to that of the infraredlight Ir_(AC) received during the period of time.

According to the technical solution described above, the disturbance toeffective signal by noise can be eliminated, and the measuring accuracyof blood oxygen content under low perfusion can be improved withoutincreasing the production cost for the measuring device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of a device formeasuring blood oxygen content;

FIG. 2 is a graph showing the curves of light absorption coefficients ofHbO₂ and Hb with respect to the regions of red light and infrared light;

FIG. 3 is a schematic diagram showing the light absorption of the tissueof an animal body;

FIG. 4 is a schematic diagram showing the comparison between the resultsof blood oxygen content measured with the area integration method andconventional method under low perfusion;

FIG. 5 is a schematic diagram showing waveforms of red light andinfrared light simulated from the sampling points;

FIG. 6 is a schematic diagram showing results of area integration of redlight and infrared light;

FIG. 7 is a block diagram showing the software modules of the measuringsystem.

DESCRIPTION OF THE PREFERED EMBODIMENTS

Now the present invention will be further described in connection withthe preferred embodiments shown in the attached figures.

According to the present invention, the disturbance to the signalwaveform by noise under low perfusion can be effectively inhibited byadopting an asymptotic integration method. It can be provedtheoretically that the asymptotic integration method is equivalent tothe conventional method for finding the AC component of waveform underlow perfusion. Thus, the asymptotic integration method is used to solvethe problem that the measured result of blood oxygen content isinaccurate under low perfusion. At least two Lights are utilized to passthrough end parts of tissues for sampling and measuring in the measuringsystem, wherein one is red light, and the other is infrared light.Firstly, the measured data of the two lights is normalized to acquirethe DC ratio of the two lights

$\left( \frac{{Ir}_{DC}}{{Red}_{DC}} \right).$The normalized waveform of blood oxygen content can be treated as thecombination of the waveform under ideal condition with noise. Thewaveform of blood oxygen content under ideal condition, both red lightand infrared light can be treated as the combination of sine waves indifferent frequency ranges, i.e.Red=a ₀ cos(ωt)+a ₁ cos(2ωt)+ . . . +a _(n−1) cos(nωt)+n _(Red)  (14)Ir=b ₀ cos(ωt)+b ₁ cos(2ωt)+ . . . +b _(n−1) cos(nωt)+n _(Ir)  (15)where a₀, a₁, . . . a_(n−1), are the first to nth components of thefrequency spectrum of red light respectively, n_(Red) is the noisecomponent in red light, b₀, b₁, . . . b_(n−1) are the first to nthcomponents of the frequency spectrum of infrared light respectively,n_(Ir) is the noise component in infrared light. The two equations aboveare integrated respectively to acquire the following ratio:

$\begin{matrix}{\frac{\begin{matrix}{\int_{t_{0}}^{t_{1}}{{{a_{0}{\cos\left( {\omega\; t} \right)}} + {a_{1}{\cos\left( {2\omega\; t} \right)}} + \cdots +}}} \\{{{{a_{n - 1}{\cos\left( {n\;\omega\; t} \right)}} + n_{Red}}}\ {\mathbb{d}\left( {\omega\; t} \right)}}\end{matrix}}{\begin{matrix}{\int_{t_{0}}^{t_{1}}{{{b_{0}{\cos\left( {\omega\; t} \right)}} + {b_{1}{\cos\left( {2\omega\; t} \right)}} + \cdots +}}} \\{{{{b_{n - 1}{\cos\left( {n\;\omega\; t} \right)}} + n_{Ir}}}\ {\mathbb{d}\left( {\omega\; t} \right)}}\end{matrix}} = \frac{{4a_{0}{\sin\left( {\omega\; t} \right)}}|_{0}^{\frac{\pi}{2}}{+ {\int_{t_{0}}^{t_{1}}{{n_{Red}}\ {\mathbb{d}\left( {\omega\; t} \right)}}}}}{{4b_{0}{\sin\left( {\omega\; t} \right)}}|_{0}^{\frac{\pi}{2}}{+ {\int_{t_{0}}^{t_{1}}{{{+ n_{Ir}}}\ {\mathbb{d}\left( {\omega\; t} \right)}}}}}} & (16)\end{matrix}$

If the noise can be treated as white noise in a period of time, theintegration of the noise will be zero. Thereby, the above equation isreduced to

$\begin{matrix}{\frac{{4a_{0}{\sin\left( {\omega\; t} \right)}}|_{0}^{\frac{\pi}{2}}{+ {\int_{t_{0}}^{t_{1}}{{n_{Red}}\ {\mathbb{d}\left( {\omega\; t} \right)}}}}}{{4b_{0}{\sin\left( {\omega\; t} \right)}}|_{0}^{\frac{\pi}{2}}{+ {\int_{t_{0}}^{t_{1}}{{{+ n_{Ir}}}\ {\mathbb{d}\left( {\omega\; t} \right)}}}}} = {\frac{a_{0}}{b_{0}} = {\frac{{Red}_{AC}}{{Ir}_{AC}}.}}} & (17)\end{matrix}$

Therefore, the ratio of AC data of the intensity of the two lights(namely, the AC peak values Red_(AC) and Ir_(AC)) received in a periodof time can be replaced with the ratio of the integration data of thetwo lights in the period of time in condition that the integrating timeis long enough to make the integration of noises approximate to zero.Furthermore, as the disturbance by noise is eliminated by the method,the measuring under low perfusion will be in well effect.

The waveforms of red light and infrared light simulated actually fromthe sampling points are shown in FIG. 5. As shown in FIG. 6, theintegrating process is performed by summing the area of the shadedportions surrounded by the curves connecting the sampling points ofpulse wave and the time axis, corresponding to red light and infraredlight respectively. When the sampling time interval between each of thesampling points is suitable (for example, when sampling at 120 Hz), thearea of the portions is approximately equal to the sum of arithmeticproduct of each sampling amplitudes and the corresponding sampling timeinterval in a period of time. And the area ratio of the two lights isapproximately equal to the AC amplitude (peak value) ratio of theintensity of the two lights in the period of time. Therefore, the bloodoxygen saturation can be calculated based on the well-known correlationbetween R and the blood oxygen saturation by measuring the AC data ofthe intensity of the two lights. The proper value of the period of timecan be selected in the range of 2 to 3 seconds based on experiences.

According to the description above, the accuracy of the measuring deviceunder low perfusion can be increased by improving the system softwareand using the method according to the present invention, based on themeasuring device as shown in FIG. 1. FIG. 7 is a block diagram showingthe system software module according to the embodiment of the presentinvention. As shown in FIG. 1, hardware initialization, systemself-check of CPU and programs initialization are first performed afterthe device is applied with power. Thereafter, a core control modulestarts a security functional module, a data processing module orcommunication functional module in accordance with the operating statusof the system correspondingly, wherein the security functional modulemeasures each of the status symbols of the system or performs systemself-check to ensure that the system can operate normally, the dataprocessing module processes the real-time collected data and thecalculated result, and the communication functional module makes thesystem receive instruction or output the data and result. Furthermore,as shown in FIG. 1, the core control module also controls the hardwarein different way under each status depending on the measured valueduring the processes of data collecting and processing, including thecontrol over driving current of light emitting diode, the control overbias circuit and gain, and the control over A/D sampling. In the presentembodiment, the process of data processing includes a process of, basedon the measured data which is collected in real time and stored in adata buffer, integrating the real-time data in a period of time tocalculate the blood oxygen saturation with the recursion process of areaintegration. Generally, the data processing module also calculates thepulse rate with a zero crossing counter, and the description thereofwill be omitted herein. The process of data processing may furtherinclude a process of fitting the envelope waveform of the pulse wave andfinding out the maximum value and the minimum value of the pulse wave tocalculate the blood oxygen saturation with the conventional waveformmethod, based on the measured data. The system may further include adecision unit. After the steps described above, the method according tothe present invention further includes a decision step in which the tworesults acquired by the data processing module with the waveform methodand the integration method respectively are decided and the finalmeasured result is generated based on the intensity of the measuredsignal.

The precondition for the decision and calculation in the decision stepis described as follows. Supposing the result of the blood oxygensaturation acquired with the waveform method is A₁, the result of theblood oxygen saturation acquired with the integration method is A₂, andthe final measured result of the blood oxygen saturation is A, thenA=a*A ₁+(1−a)*A ₂,where the value of “a” can be selected in the range of 1 to 0 dependingon the intensity of the measured signal.

The measured signal with relative high intensity, for example, thecollected analog signal whose intensity can reach the full range of A/Dconverting without being amplified, is taken as a reference. If theintensity of actual measured signal is lager than one thirty-second (1/32) of the intensity of the reference, the results of A₁ and A₂ becomeapproximate to each other, thereby the value of “a” can be selected as0.5 (i.e. a=0.5), and the average value of A₁ and A₂ is adopted as thefinal result; if the intensity of actual measured signal is smaller thanone thirty-second ( 1/32) of the intensity of the reference but largerthan one sixty-forth ( 1/64) of the intensity of the reference, thevalue of “a” can be selected as 0.4 (i.e. a=0.4); if the intensity ofactual measured signal is smaller than one sixty-forth ( 1/64) of theintensity of the reference but larger than one one-hundred-twenty-eighth( 1/128) intensity of the reference, the value of “a” can be selected as0.3 (i.e. a=0.3); and the like. If the intensity of actual measuredsignal decreases to a certain degree, the value of “a” can be selectedas 0 (i.e. a=0), and the result acquired with the integration method isadopted as the final result.

The precondition of the method described above is that the blood oxygensaturation of the measured object is constant in a period of time. Inthis case, the longer integrating time results in better measuringeffect and then more close to the truth. But once the blood oxygensaturation of the measured object changes (generally changes gently),the overlong integrating time may result in the decrease of measuringsensibility, thereby the real-time measuring or monitoring function ofthe system is degraded. To solve this problem, during the processesdescribed above, the integration is only performed in a period of time(for example, 2-3 seconds), and a forgetting factor λ is incorporated toretain the real-time monitoring function. Therefore, the ratio of the ACpeak value Red_(AC) and Ir_(AC) of the current two lights becomes

$\begin{matrix}{{\frac{{Red}_{AC}}{{Ir}_{AC}} = \frac{{Red}_{{AC}_{0}} + {\lambda\;{Red}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Red}_{{AC}_{n}}}}{{Ir}_{{AC}_{0}} + {\lambda\;{Ir}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Ir}_{{AC}_{n}}}}},} & (18)\end{matrix}$

where Red_(AC) ₀ , Ir_(AC) ₀ are the results of the area integration ofcurrent time, Red_(AC) ₁ , Ir_(AC1) are the results of the areaintegration of last time, and Red_(AC) _(n) , Ir_(AC) _(n) are theresults of the area integration of the former nth time. If theforgetting factor ranges from 0 to 1 (i.e. 0<λ<1), the effect of thedata measured more formerly to the calculated result of current time canbe ignored after iteration for several times. Therefore, the data, whichis calculated more newly for the current time, makes more contributionto the current result. Based on experiences, it is appropriate to selectthe value of λ as 0.8.

FIG. 4 is a schematic diagram showing the comparison between themeasured results with the method according to the present invention andthe conventional method, wherein the ordinate is blood oxygen saturationin the measuring range of 0 to 100, and the abscissa is time. As seen inFIG. 4, though the undulation of initial measured data is relativelylarge, the undulation of the measured result with the method accordingto the present invention is smaller than that of the measured resultwith the conventional method during normally monitoring. That is to say,the resistance to noise disturbance is getting improved. According tothe present invention, it is possible to increase greatly the measuringaccuracy of blood oxygen content under low perfusion without increasingthe production cost for the measuring device. In particularly, the bloodoxygen signal with the intensity of 0.3% can be accurately measured withthe conventional method, while the blood oxygen signal with theintensity of 0.1% can be accurately measured with the method accordingto the present invention.

1. A method for measuring blood oxygen content under low perfusion,which is used in a device for measuring blood oxygen content, includingthe steps of: a. initializing the device that is applied with power; b.collecting and processing data with a driving circuit of a lightemitting device, a bias circuit, a gain circuit and an A/D samplingcircuit, which are controlled under a core control module; c.calculating blood oxygen saturation based on the collected data with adata processing module which integrates the collected data in a periodof time with an area integration method, wherein the collected data iscyclical and the period of time for integration includes more than onecycle; and d. outputting from a communication functional module a resultof the blood oxygen saturation or pulse rate calculated with the dataprocessing module.
 2. The method for measuring blood oxygen contentunder low perfusion according to claim 1, wherein in the step of c, thedata processing module further fits envelope waveform of a pulse waveand finds out the maximum value and minimum value of the pulse wave tocalculate the blood oxygen saturation with a waveform method based onthe collected data.
 3. The method for measuring blood oxygen contentunder low perfusion according to claim 2, wherein the method, betweenthe steps of c and d, further includes a decision step of decidingbetween the two results acquired from the data processing module withthe waveform method and the integration method respectively based on theintensity of measured signals and generating a final measured result,performed by a decision unit included in the device.
 4. The method formeasuring blood oxygen content under low perfusion according to claim 3,wherein a precondition for the decision and calculation in the decisionstep is:A=a*A ₁+(1−a)*A ₂, where “A₁” is the result of the blood oxygensaturation acquired with the waveform method, “A₂” is the result of theblood oxygen saturation acquired with the integration method, “A” is thefinal measured result of the blood oxygen saturation, and “a” rangesfrom 1 to
 0. 5. The method for measuring blood oxygen content under lowperfusion according to claim 2, wherein the device performs the samplingand measuring by making two lights pass through end parts of tissues,wherein one is red light, and the other is infrared light; and in thestep of c, the data of the two lights is integrated respectively tocalculate the ratio of integration result of the red light to that ofthe infrared light for replacing the ratio of AC peak value of theintensity of the red light Red_(AC) to that of the infrared lightIr_(AC) received during the period of time.
 6. The method for measuringblood oxygen content under low perfusion according to claim 5, wherein:in the step of c, the calculation of the data processing module alsodepends on a forgetting factor λ which ranges from 0 to 1, and the ratioof the AC peak value Red_(AC) to Ir_(AC) of the current two lights is${\frac{{Red}_{AC}}{{Ir}_{AC}} = \frac{{Red}_{{AC}_{0}} + {\lambda\;{Red}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Red}_{{AC}_{n}}}}{{Ir}_{{AC}_{0}} + {\lambda\;{Ir}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Ir}_{{AC}_{n}}}}},$where Red_(AC) ₀ , Ir_(AC) ₀ are the results of the area integration ofcurrent time, Red_(AC) ₁ , Ir_(AC) ₁ are the results of the areaintegration of last time, and Red_(AC) _(n) , Ir_(AC) _(n) are theresults of the area integration of a former nth time.
 7. The method formeasuring blood oxygen content under low perfusion according to claim 6,wherein the value of the forgetting factor λ is 0.8.
 8. The method formeasuring blood oxygen content under low perfusion according to claim 3,wherein the device performs the sampling and measuring by making twolights pass through end parts of tissues, wherein one is red light, andthe other is infrared light; and in the step of c, the data of the twolights is integrated respectively to calculate the ratio of integrationresult of the red light to that of the infrared light for replacing theratio of AC peak value of the intensity of the red light Red_(AC) tothat of the infrared light Ir_(AC) received during the period of time.9. The method for measuring blood oxygen content under low perfusionaccording to claim 8, wherein: in the step of c, the calculation of thedata processing module also depends on a forgetting factor λ whichranges from 0 to 1, and the ratio of the AC peak value Red_(AC) toIr_(AC) of the current two lights is${\frac{{Red}_{AC}}{{Ir}_{AC}} = \frac{{Red}_{{AC}_{0}} + {\lambda\;{Red}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Red}_{{AC}_{n}}}}{{Ir}_{{AC}_{0}} + {\lambda\;{Ir}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Ir}_{{AC}_{n}}}}},$ where Red_(AC) ₀ , Ir_(AC) ₀ are the results of the area integration ofcurrent time, Red_(AC) ₁ , Ir_(AC) ₁ are the results of the areaintegration of last time, and Red_(AC) _(n) , Ir_(AC) _(n) are theresults of the area integration of a former nth time.
 10. The method formeasuring blood oxygen content under low perfusion according to claim 9,wherein the value of the forgetting factor λ is 0.8.
 11. The method formeasuring blood oxygen content under low perfusion according to claim 4,wherein the device performs the sampling and measuring by making twolights pass through end parts of tissues, wherein one is red light, andthe other is infrared light; and in the step of c, the data of the twolights is integrated respectively to calculate the ratio of integrationresult of the red light to that of the infrared light for replacing theratio of AC peak value of the intensity of the red light Red_(AC) tothat of the infrared light Ir_(AC) received during the period of time.12. The method for measuring blood oxygen content under low perfusionaccording to claim 11, wherein: in the step of c, the calculation of thedata processing module also depends on a forgetting factor λ whichranges from 0 to 1, and the ratio of the AC peak value Red_(AC) toIr_(AC) of the current two lights is${\frac{{Red}_{AC}}{{Ir}_{AC}} = \frac{{Red}_{{AC}_{0}} + {\lambda\;{Red}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Red}_{{AC}_{n}}}}{{Ir}_{{AC}_{0}} + {\lambda\;{Ir}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Ir}_{{AC}_{n}}}}},$where Red_(AC) ₀ , Ir_(AC) ₀ are the results of the area integration ofcurrent time, Red_(AC) ₁ , Ir_(AC) ₁ are the results of the areaintegration of last time, and Red_(AC) _(n) , Ir_(AC) _(n) are theresults of the area integration of a former nth time.
 13. The method formeasuring blood oxygen content under low perfusion according to claim12, wherein the value of the forgetting factor λ is 0.8.
 14. The methodfor measuring blood oxygen content under low perfusion according toclaim 1, wherein the device performs the sampling and measuring bymaking two lights pass through end parts of tissues, wherein one is redlight, and the other is infrared light; and in the step of c, the dataof the two lights is integrated respectively to calculate the ratio ofintegration result of the red light to that of the infrared light forreplacing the ratio of AC peak value of the intensity of the red lightRed_(AC) to that of the infrared light Ir_(AC) received during theperiod of time.
 15. The method for measuring blood oxygen content underlow perfusion according to claim 14, wherein: in the step of c, thecalculation of the data processing module also depends on a forgettingfactor λ which ranges from 0 to 1, and the ratio of the AC peak valueRed_(AC) to Ir_(AC) of the current two lights is${\frac{{Red}_{AC}}{{Ir}_{AC}} = \frac{{Red}_{{AC}_{0}} + {\lambda\;{Red}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Red}_{{AC}_{n}}}}{{Ir}_{{AC}_{0}} + {\lambda\;{Ir}_{{AC}_{1}}} + \cdots + {\lambda^{n}{Ir}_{{AC}_{n}}}}},$where Red_(AC) ₀ , Ir_(AC) ₀ are the results of the area integration ofcurrent time, Red_(AC) ₁ , Ir_(AC) ₁ are the results of the areaintegration of last time, and Red_(AC) _(n) , Ir_(AC) _(n) are theresults of the area integration of a former nth time.
 16. The method formeasuring blood oxygen content under low perfusion according to claim15, wherein the value of the forgetting factor λ is 0.8.
 17. The methodfor measuring blood oxygen content under low perfusion according toclaim 1, wherein in the step of c, the period of time for integration is2-3 seconds.
 18. The method for measuring blood oxygen content under lowperfusion according to claim 1, wherein the integration determines anarea between the collected data and a normalized value of the collecteddata.